This invention relates to electric vehicles such as fork lift trucks and tow tractors and discloses a temperature regulation system for use on such a vehicle which is intended to be operated in extremely cold environments as well as extremely warm environments.
Fork lift trucks have been known for some time. Examples of fork lift trucks are shown in U.S. Pat. Nos. 3,637,100 and 3,532,238. Both of the latter two patents are incorporated by reference herein. While fork lift trucks of the type described in the above-cited patents are very useful for most applications, special problems arise when such trucks are to be used in an extremely cold environment, such as a freezer building used to store frozen food. Fork lift trucks are used to transfer frozen food between a freezer warehouse and a large freezer road truck. A vehicle operating in such a freezer warehouse must be able to operate satisfactorily at temperatures as low as about -30.degree. F., while at the same time being able to adapt quickly to operation in outside temperatures, which may be 90.degree. F. or higher on hot summer days.
When a fork lift truck operates in a cold environment, water tends to condense on the truck (the truck being warmer than its surroundings) and this condensation causes eventual corrosion or freeze up of the electrical equipment on the truck. Especially vulnerable are components such as the Silicon Controlled Rectifier (SCR) used in the electrical control system for fork lift vehicles of the type shown in the above-cited patents.
Fork lift trucks are sometimes designed such that all, or almost all, of the electrical components are physically located together, in a so-called electrical box. In such a case, it is important that the electrical box be kept dry, to prevent corrosion. But is is also important that the temperature of the electrical box not become too high, as would tend to happen when the truck is driven out of the freezer warehouse and into a warm environment.
It is also necessary to heat other components of the fork lift truck. For example, the accelerator for the vehicle may comprise a set of components mounted in a box, as shown in U.S. Pat. No. 4,087,776. This accelerator box needs to be kept dry. Also, the control console of the vehicle typically contains a set of components, located in one place, and also needs protection from condensation.
In the past, one solution to the problem of operating an electric vehicle in an extremely cold environment has been to place electric resistance heaters at selected locations around the vehicle. The components of, say, the electrical box could be kept warm and dry in this manner, at the expense of electrical energy needed to supply the heat. In addition to requiring large amounts of energy, resistance heaters will tend to produce too much heat when the vehicle is driven into a warm environment, since the resistance heat is added to the heat in the ambient air.
Electric fork lift vehicles are typically driven by DC motors, which have efficiencies of about 85%, under no load conditions, and about 70% in a loaded condition. The remaining energy is dissipated as heat. This fact is particularly ironic in the context of the vehicles for operation in cold environments, since heat is precisely what is needed at certain key positions around the truck, to keep components warm and dry.
The present invention solves the problem of keeping critical components warm and dry while the vehicle is in a cold environment, and does so in a manner which conserves energy Furthermore, the invention allows the vehicle to adjust to the vast difference in ambient conditions between the inside of a freezer and normal outside temperatures.